The present invention relates to a radio frequency (RF) resonator and an apparatus using a RF resonator circuit. More specifically, the invention relates to an ion-trapping device namely a quadrupole ion trap.
The quadrupole ion trap is initially described by Paul et al in U.S. Pat. No. 2,939,952 and normally consists of three electrodes; a ring electrode and two end-cap electrodes one on each side of the ring electrode. A radio frequency (RF) voltage is normally applied to the ring electrode, and two end-cap electrodes are normally grounded. A coil is connected to the ring electrode forming a LC resonator together with the capacitance between the ring electrode and the two end-cap electrodes as well as the capacitance of all other circuit elements connected to the ring electrode. Because of the high Q-value of the resonator, even a low voltage RF driver, which is connected to the coil directly or through transformer coupling, can effectively produce a high RF voltage on the ring electrode.
A quadrupole ion trap can be used as an ion-trapping device of mass analysis apparatus together with a variety of ion sources. One of the most popular ion sources is liquid chromatography with electrospray ionization. Another ion source, which seems a very promising combination with the ion trap, is matrix-assisted laser desorption/ionization (MALDI).
The ions produced by a MALDI ion source are inevitably pulsed and synchronized to the laser pulse. A trapping efficiency of those ions is maximized by using a method of fast start of RF voltage as described in UK Patent Application No.9802112.4. Unfortunately, an ordinary RF driver circuit provides only a low excitation voltage, and the rate of increase of RF voltage becomes quite low. The time to establish a required RF voltage usually requires several cycles of RF or more than that.
Another attempt was made using a quadrupole ion trap in conjunction with a time-of-flight mass spectrometer, which is described in the UK Patent Application No.9802111.6. In this method ion extraction voltages are applied to the two end-cap electrodes, and the RF voltage on the ring electrode should be terminated at the same time or nearly the same time of application of the extraction voltages. To prevent the ions escaping from the ion-trapping region, the termination should be fast enough, at least less than one cycle of the RF. Again, this is difficult to achieve using an ordinary RF driver circuit. While the excitation voltage of the RF driver circuit is terminated, the energy stored in the resonator is consumed and is released only via the resistance of the resonator, and the time constant of decay of the RF voltage is usually much more than one cycle of the RF.
It is an object of the invention to provide a method of fast start of the RF resonator and/or fast termination of the resonator to achieve a high efficiency of ion trapping from the MALDI ion source and/or effective extraction of the ions into a time-of-flight mass spectrometer.
Accordingly, the invention provides a method of fast start and/or fast termination of a radio frequency resonator, which has a coil, a capacitor means and two switch means each having an internal resistance, one end of each said switch means being connected to a junction of said coil and said capacitor means where a radio frequency voltage is provided, and another end of each said switch means being connected to high voltage power supplies having opposite polarities, the method comprising: closing one of said switch means for a short period of time for fast start of said radio frequency resonator, and/or closing both of said switch means for a time interval for fast termination of said radio frequency resonator.
In the MALDI ion source, ions are produced by a laser pulse directed at the sample surface, and are guided into an ion trap using ion optics built inside the MALDI ion source. Before ions are produced, the RF voltage applied on the ring electrode is zero. When the ions are inside the ion-trapping region surrounded by three electrodes, one of the switches is closed to charge the capacitance between the ring electrode and the end-cap electrodes, the capacitance due to additional circuitry, for example a measuring circuit of the RF voltage, and a stray capacitance. Once the capacitance is charged, the switch is opened immediately to initiate a free oscillation of the resonator. The excitation voltage of the RF driver circuit is also set to a proper voltage to sustain the oscillation, but timing is not so critical because the RF voltage does not change so rapidly with a lower excitation voltage of the RF driver circuit.
For extraction of ions into a time-of-flight mass spectrometer, the RF voltage is turned to zero by closing both switches at the same time. When the high voltage power supplies connected to the switches have the same voltage with opposite polarities and the switches have the same internal resistance the RF voltage applied between the two switches approaches to zero with a time constant determined by the capacitance between the electrodes and the resistance of the switches. This time constant can be set small enough to prevent the ions escaping from the ion-trapping region. The extraction voltages are also applied to the end-cap electrodes at nearly the same time as the RF termination for extracting ions into the time-of-flight mass spectrometer. The switches are kept closed until all the ions are extracted from the ion trap. Later, the switches are opened, without affecting ion motion. The excitation voltage of the RF driver circuit is also set to zero to prevent oscillation after ion extraction, but again timing is not so critical because the RF voltage is kept zero by the switches while ions are extracted.